Officials said a thorough, block-by-block investigation since Friday's blast indicates the city's gas system is safe, intact and functioning properly.

The probe will now focus on the Columbia Gas employee responsible for puncturing a gas line with a tool, and whether proper protocol was followed in the build-up to Friday's explosion. The blast sent firefighters and others to the hospital, damaged more than three dozen buildings, and prompted a large-scale evacuation of the city's downtown entertainment district.

"Human error ... is what the cause of the explosion was," Coan said Sunday, attributing the origins of the blast to a utility worker who punctured an underground gas line with a metal probing tool.

A strong smell of natural gas that was detected in the Worthington Street area late Friday afternoon prompted the evacuation of much of the city's entertainment district as Columbia gas crews searched for the source of the leak.

Coan said the gas company worker inadvertently punctured the pipe, causing gas to escape into Scores Gentlemen's Club. The volume of gas inside the multistory brick building at 453 Worthington St. reached such a high level, that any number of possible ignition sources could have triggered the explosion, Coan said.

Street markings showing the supposed location of underground gas lines were inaccurate, Coan said, adding that further investigation by the state Department of Public Utilities (DPU) will focus on why that was the case. State Energy and Environmental Affairs Secretary Richard K. Sullivan Jr., who oversees the DPU, will ultimately serve as point man for the probe at this point.

"We are transitioning the investigation to the DPU," Coan said, speaking to reporters at a Sunday afternoon news conference at Springfield Fire Department headquarters on Worthington Street.

Coan said Columbia Gas officials have assured him that Springfield's natural gas system is functioning properly and poses no threat to the public. Officials did not publicly identify the worker who damaged the line, nor did they indicate if he's facing any disciplinary action.

The worker began searching outside the building, using the metal probe to make holes in order to measure gas levels. His examination appears to have been an appropriate distance from where older markings on a sidewalk indicated the gas line was buried. However, the markings were incorrect and his metal probe inadvertently punctured the high-pressure gas line near the foundation of the Scores building, according to Coan.

Sullivan said the DPU will focus on the utility company's protocols and regulations. "The company has fully cooperated, giving us access to all of their employees for interviews," Sullivan said. "Now, we will be looking at issues of possible regulation violations as well as the conduct of employees before, during and after the explosion."

Sullivan said investigators will examine whether all necessary resources were provided in a timely manner. But they will specifically focus on the issue of the incorrect street markings, which led to the worker rupturing the underground line. "We will be conducting a thorough investigation that may not be done in days, but hopefully in weeks. We will move as quickly as possible," the energy secretary said.

Friday's 5:25 p.m. blast leveled Scores and shut down much of the city's entertainment district, with some sections remaining off limits until further notice. Worthington Street is expected to be closed between Chestnut and Spring streets for at least several more days, and yellow caution tape now adorns numerous damaged facades within the blast zone near the corner of Worthington and Chestnut streets.

Sections of the city's normally bustling entertainment district on Sunday looked more like war-torn parts of Belfast or Beirut in the bad old days, with glass and twisted chunks of metal strewn about and pushed into makeshift piles cordoned-off by yellow tape. The explosion damaged 42 buildings housing 115 residential units. Three buildings were immediately condemned, and 24 others required additional inspections by structural engineers to determine whether they are safe.

Twelve of the 14 Springfield firefighters who responded to the initial gas leak report were injured in the explosion, according to Springfield Fire Commissioner Joseph A. Conant. "We're hoping that our firefighters recover quickly and get back to work," Conant said.

Firefighters sustained everything from knee injuries to bruises to concussions from the impact of the blast. Two firefighters who suffered severe burns to their faces and heads will likely take longer to recover, Conant said. "It could have been far worse than it was," the commissioner said.

Just hours before Friday's blast, downtown Springfield was filled with families as they watched the city's annual Parade of Big Balloons under a clear, blue sky with unseasonably mild temperatures.

All told, 21 people were injured, including the firefighters. Among the wounded were Columbia Gas workers, city police officers and a photo journalist. None of the injuries were life-threatening, officials said.

"Again, this has been a terrible tragedy," said Mayor Domenic J. Sarno, whose administration last year was tested by a devastating tornado and a powerful northeaster. Friday's blast was yet another reminder that Springfield is now adept at handling large-scale disasters, he said.

"It was a miracle on Worthington Street that no one was killed," the mayor said, echoing a line first uttered by Lt. Gov. Timothy P. Murray on the evening of the blast.

Sarno said Columbia Gas officials have been "extremely cooperative," making themselves available at every step of the investigation. Utility officials also have vouched for the safety of the gas system after they conducted an extensive assessment of the infrastructure, the mayor said. "They have taken responsibility," Sarno said.

Conant said Friday's incident began with a 4 p.m. report involving a strong odor of gas in the area of Worthington and Chestnut streets. At 4:09 p.m., a Columbia Gas employee arrived on scene and began looking for the source of the odor. At 4:20 p.m., the employee punctured a hole in an underground pipe and called for assistance.

The area was evacuated once it was clear that there was a gas leak, Conant said. The gas company took between 20 to 30 minutes to arrive to shut off the gas. The explosion occurred around 5:25 p.m., Conant said. The fire commissioner would not comment on whether a half-hour is the standard response time for a gas leak, or longer than normal.

Columbia Gas will be opening a temporary claims center on Monday at Springfield City Hall. The center will be open from 10:30 a.m. to 5 p.m. in Room 222.